In the manufacture of molded records, such as conventional audio records or the more recently developed capacitive electronic discs, a thermoplastic composition is molded between a pair of metal disc-shaped parts referred to as stampers. Each of the stampers has defined in its molding surface a spiral information track which contains a surface relief pattern corresponding to the program information desired to be reproduced on playback of a molded record pressed with the stampers.
Stampers are the end product of a multi-step process which is broadly referred to as matrixing. The first step of the matrixing process is to record the program information on a magnetic tape. The magnetic tape is used to control a tool which cuts a recording substrate with a spiral information track having a surface relief pattern corresponding to the recorded program information. The recording substrates which are conventionally employed are flat metal discs which have a layer of a relatively soft material formed on one surface thereof into which the spiral information track is cut.
The recording substrate having the spiral information track cut into its surface is replicated in a series of electroforming steps. In the first of the electroforming steps, a metal, such as nickel, is electrodeposited on the surface of the recording substrate until a self-substaining part of a predetermined thickness is obtained. The resulting electroformed part, called a master, is separated from the recording substrate. The master is, in turn, replicated by electrodepositing a metal, again, such as nickel, onto the surface of the master. After a predetermined thickness of metal has been deposited on the master, the resulting replica, referred to as a mother, is separated from the surface of the master. The mother which is obtained is, in turn, replicated to produce the part referred to as a stamper. The stamper which is obtained is a negative replica of the recording substrate having formed in its molding surface a mirror image of the spiral information track which was cut into the recording substrate and is a flat disc-shaped part like the recording substrate.
The stampers obtained directly from the matrixing process described above are not suitable as formed for use with the presses conventionally employed in the molding of records. The flat stampers obtained from the matrixing process could more properly be called stamper blanks in that they must be subjected to a number of edge shaping steps in order to have the required edge configuration for mounting on a molding platen of a record molding press. The edges of flat stampers from the matrixing process are initially trimmed. An inner edge is formed in the flat stampers by cutting a center hole into the flat stampers of a precise diameter concentric with the recorded area of the stampers. The outer edge of the flat stampers is also trimmed to a precise diameter concentric with the recorded area of the stampers. After the stampers are trimmed, the inner and outer edges are further shaped in stamping operations. The inner edge is stamped so as to form a tube-like extension about the inner edge which is used to secure the center portion of the stamper to the molding platen. The outer edge of the stamper is likewise shaped in a stamping operation which results in the outer edge being shaped so as to form an outer bead on the molded record as well as to form the flash and provide a land area for the molding process. In addition, a circumferential portion is formed about at the outermost diameter for engaging the outer edge of the molding platen.
The requirements for shaping of the inner and outer edges of the stampers are a source of considerable problems. It is highly desirable that the stampers be as hard as possible so as to resist scratching, dents and other types of mechanical damage during handling and pressing. However, since the flat stampers from the matrixing process are required to be shaped in stamping operations, as noted above, it is necessary that the metal of the stampers be sufficiently ductile to permit deformation without cracking when the stampers are shaped, or, thereafter, when they are used in the molding process. The requirement for ductility of stampers manufactured by the above-described matrixing process has resulted in an undesirable trade-off of hardness in order to improve ductility. Furthermore, it has been found that despite compromises being made in the hardness of the stampers in order to obtain increased ductility, almost all of the problems encountered with the stampers during the pressing of records can be traced directly back to the stamping steps used to shape the inner and outer edges. The stampers, prepared as described above, when used in the molding process, usually fail as a result of fatigue cracking at the shaped inner or outer edges of the stampers. This is believed to be due to stresses introduced into the shaped inner and outer edges during stamping.
What would be highly desirable would be an improved method for the manufacture of stampers having increased hardness in the recorded areas and increased resistance to fatigue cracking in the shaped inner and outer edges.